Energy is a quantity that has many forms, with electrical energy having the attractive property that it may be easily transmitted through conductors and thereby transferred to remote locations without the requirement for mechanical transport. Electrical energy may be used to generate mechanical motion through motors, and can power sensors, electronics and heaters. A less versatile manifestation of energy is thermal energy, or heat. Thermal energy may be produced as a byproduct of exothermic chemical reactions, such as the combustion of hydrocarbons in an internal combustion engine. Thermal energy may be derived from hot springs and vents in the earth's crust (geothermal energy) and as a byproduct of radioactive decay. Thermal energy may also be collected as the absorption of solar radiation.
Thermoelectric generation is the conversion of thermal energy into electrical energy without the intermediary of rotating machinery. Status quo thermoelectric generators are constructed from thermoelectric elements that are generally formed of doped semiconductors. Even the best of these suffers from inefficiencies that render them unusable for all but a limited class of applications. Transport mechanisms in solid state materials are complicated, interrelated and require detailed quantum mechanical calculations to describe. However, for the purposes of the following discussion we will focus on the effect of lattice vibrations on the thermoelectric qualities of a material. These mechanical excitations of the atoms in a solid are called phonons and they have two main undesirable effects in a thermoelectric material. The first effect is that they transmit heat directly through a material, effectively acting as a leak for thermal energy. The second effect is to scatter electrons as they travel through the material, effectively causing electrical resistance and joule heating. The present invention describes a fabrication method using shockwave powder consolidation that reduces lattice vibrations, leading to an improved thermoelectric performance.
Thermoelectric generation takes place when a temperature difference is applied to a conductor or semiconductor and causes mobile charge carriers, either electrons or holes, to migrate from hot to cold. The resulting separation of charge creates an electric field potential known as the Seebeck voltage, that is given by ΔV=αΔT, where α is a temperature dependent material property known as the Seebeck coefficient and, by convention, ΔT represents the temperature of the cold side with respect to the hot side. The Seebeck coefficient for a material may be positive or negative depending upon the type of majority charge carrier.
For thermoelectric devices operating around room temperature (300K), it is common to use doped alloys of tellurium, as the active elements for converting thermal energy to electrical energy. Doped bismuth-telluride (BiTe) alloys are the most popular for a temperature range of about 220K to 420K and have the attractive properties of a relatively high Seebeck coefficient α, a relatively high electrical conductivity, σ, and a relatively low thermal conductivity λ=λe+λl where λe=LTσ is the portion of thermal conductivity due to electrons with T being temperature and L being the Lorentz constant, and λl being the portion of thermal conductivity due to lattice vibrations (phonons). These bulk material properties are often lumped into a single figure of merit Z, where
                    Z        =                                            σα              2                                                      λ                e                            +                              λ                1                                              .                                    (        1        )            In general, the higher the value of Z, the better the thermoelectric conversion efficiency. The parameters α, σ, and λl are temperature dependent.
Thermoelectric materials have traditionally been manufactured through a bulk process. Bulk fabrication techniques include melting and powder 10 compaction. However, for any given material, the constituents of the figure-of-merit, Z, namely, α, λ, and S, are tightly intercoupled, so that changes that are made in chemistry or crystalline structure that influence one of these parameters in a positive way are generally offset by a negative influence on another parameter. By contrast, device fabrication on the nanoscale (with feature sizes under 20 nanometers) alters the relationship between the various constituents of Z, enabling another variable for optimization.
Z is not the only important metric for analyzing a TE device and is, at best, an average quantity. Often, thermoelectric materials are discussed in terms of the dimensionless product ZT in order to have a common reference point when discussing materials that have been optimized over different temperature ranges.
There must be an electron flow in a thermoelectric generator since the object is to supply electrical current. As such, electronic heat transfer is unavoidable and most of the focus in reducing thermal conduction has been directed at reductions in lattice (phonon) transport. Given a specific chemical make-up, thermoelectric materials may be fabricated as a single crystal, in polycrystalline form or as an amorphous (non crystalline) form. In electrically conductive substances, crystals are generally good material structures for both electron and heat transmission. Their regular structure promotes long mean free paths (mfp's) which are the mean distances that an electron or phonon travels between collisions. In contrast with a crystal, a glass exhibits no ordering between molecules and is said to have an amorphous structure. Accordingly, it has been proposed that a desirable property for a thermoelectric will be that it resembles a phonon glass and an electron crystal [G. A. Slack in CRC Handbook of Thermoelectrics, CRC Press, 1995, p. 411]. Much research has been invested into methods that scatter phonons (thus reducing their mean free path) more effectively than electrons.
One approach to building low thermal conductivity materials is to use powder sintering. The constituent materials are ground into a powder, then combined by compaction and sintering (heating). These constituent materials may be provided as individual powdered elements and then mixed and compacted and sintered. Alternately, the constituents may be prepared as a melt and then ground into a powder that is compacted and sintered. The approach of powder compaction and sintering is said to introduce disorder, lattice defects and grain boundaries which will inhibit phonon transport without excessively compromising electron transport.
U.S. Pat. No. 3,524,771 to Green describes a method for preparing thermoelectric materials consisting of small particles that are sintered to form a solid element. This approach is said to reduce thermal conductivity but at the expense of reduced electrical conductivity. U.S. Pat. No. 5,411,599 to Horn et al, describes a technique for fabricating thermoelectric materials with low thermal conductivity whereby a nanoporous structure is achieved by chemically preparing particles of a bismuth telluride based alloy and then compacting these particles. The resulting device is said to have nanoinclusions which lead to a reduced phonon conductivity. U.S. Pat. No. 6,319,744 B1 to Hoon et al., describes a technique for manufacturing thermoelectric semiconductor material by laminating strips of thin powders and then compressing and sintering to form a composite solid. U.S. Pat. No. 6,596,226 B1 to Simard describes a compaction method for alloying constituent powders in order to devise thermoelectric devices. The procedure consists of mechanically alloying the constituent elements in a powder form, compacting the resulting powder, heat treating the alloy and then extruding the resultant device. U.S. Pat. No. 7,365,265 B1 to Heremans et al, describes a technique to build thermoelectric elements using a nanogranular material which is compressed and sintered.
It is important to note that all previously proposed techniques for constructing thermoelements from powders have utilized compaction techniques as opposed to explosive consolidation. Although samples prepared using the two techniques can have identical particle densities, a consolidated sample will have maximum particle to particle bonding [K. P. Staudharnmer and L. E. Murr, “Principles and applications of shock wave compaction and consolidation of powdered materials”, in Shock Waves for Industrial Applications, L. E. Murr, Editor, Noyes Publications, 1988, p. 2381. Also, when powder compaction is combined with sintering, the sintering occurs at comparably large temperatures, effectively annealing the particles. Explosive consolidation is, in effect, a relatively low temperature procedure that is not conducive to crystal formation.
U.S. Pat. Nos. 4,717,627 and 4,907,731 to Nellis et al describe a shockwave consolidation approach to the fabrication of fine grain materials having desirable superconducting or magnetic properties. U.S. Pat. No. 5,129,801 to Rabin et al describes an explosive consolidation for powders which uses an explosively propelled piston to compress a pelletized powder with particular applications to titanium carbide and alumina. These inventions propose shockwave consolidation as a means to fabricate homogeneous monoliths and do not suggest the thermoelectric advantages that will accrue from a reduction in thermal transport.